Silicon carbide (SiC) is a high-hardness semiconductor material with a greater bandgap than silicon (Si), and has been used extensively in various types of semiconductor devices including power elements, hostile-environment elements, high temperature operating elements, and radio frequency elements. Among other things, the application of SiC to power elements such as semiconductor elements and rectifiers has attracted a lot of attention. This is because a power element that uses SiC can significantly reduce the power loss compared to a Si power element. In addition, by utilizing such properties, SiC power elements can form a smaller semiconductor device than Si power elements.
A metal-insulator-semiconductor field effect transistor (MISFET) is a typical semiconductor element among various power elements that use SiC. In this description, a MISFET of SiC will sometimes be simply referred to herein as a “SiC-FET”. And a metal-oxide-semiconductor field effect transistor (MOSFET) is one of those MISFETs.
Somebody reported that if a forward current is supplied to the pn junction of SiC, stacking defects will increase due to basal plane dislocations, which is a problem unique to SiC. Such a problem will arise when a SiC-FET is used as a switching element for a power converter for driving and controlling a load such as a motor. If a SiC-FET is used as a switching element for a power converter, then return current needs to flow when the SiC-FET is in OFF state. And a pn junction inside of a SiC-FET is sometimes used as a path for such a return current. Such a pn junction is present deep inside a semiconductor element that works as the SiC-FET and functions as a kind of diode. That is why the pn junction is called a “body diode”. If the pn junction diode (body diode) inside a SiC-FET is used as a freewheeling diode, then current will flow in the forward direction through the body diode that is a pn junction. It is believed that if such current flows through the pn junction of SiC, then the degree of crystallinity of a SiC-FET will decrease (i.e., the number of stacking defects at the pn junction will increase) due to a bipolar operation performed by the body diode (see, for example, Patent Document No. 1 and Non-Patent Documents Nos. 1 and 2).
If the degree of crystallinity of a SiC-FET decreases, the ON voltage of the body diode could rise. Also, if a body diode is used as a freewheeling diode, a reverse recovery current will flow due to the bipolar operation performed by the pn junction diode when the diode in ON state changes into OFF state. And that reverse recovery current causes not only recovery loss but also a decrease in switching rate as well.
Thus, in order to overcome such a problem involved by using a body diode as a freewheeling diode, it was proposed (in Patent Document No. 2, for example) that a return current is made to flow through a freewheeling diode element as an electronic part by connecting the freewheeling diode element and an SiC-FET in anti-parallel with each other.
FIG. 8 illustrates a configuration for a typical inverter circuit 1000 including freewheeling diode elements.
The inverter circuit 1000 is a circuit for driving a load 1500 such as a motor and includes multiple semiconductor elements 1100, which are implemented as SiC-FETs. In this inverter circuit 1000, the semiconductor elements 1100 and freewheeling diode elements 1200 are connected in anti-parallel together. on-state current IF flows through the semiconductor elements 1100 and return current IR flows through the freewheeling diode elements 1200. Two semiconductor elements 1100 that are connected in series together form one set of semiconductor elements. And three sets of semiconductor elements are arranged in parallel with each other with respect to a DC power supply 2000. The gate electric potential of each of these semiconductor elements 1100 is controlled by a controller.
FIG. 9 illustrates the structure of a semiconductor element (SiC-FET) 1100. This semiconductor element 1100 is made of silicon carbide (SiC) semiconductors and has a structure in which an n−-silicon carbide semiconductor layer 120 has been stacked on an n+-substrate (SiC substrate) 110. A p-body region 130 has been defined in an upper part of the n−-silicon carbide semiconductor layer 120. A p-body contact region 132 and an n+ impurity region 140 have been defined in an upper part of the p-body region 130. And a first ohmic electrode 145 is arranged on the p-body contact region 132 and the n+-impurity region 140.
An epitaxial channel layer 150 has been formed to cover the surface of the n− silicon carbide semiconductor layer 120, the p-body region 130 and the n+-impurity region 140. A gate insulating film 160 and a gate electrode 165 are further arranged on the epitaxial channel layer 150. A portion of the epitaxial channel layer 150, which contacts with the upper surface of the p-body region 130, functions as a channel region. On the back surface of the n+-substrate 110, arranged is a second ohmic electrode 170.
A body diode 180 has been formed inside of this semiconductor element 1100. Specifically, the pn junction between the p-body region 130 and the n−-silicon carbide semiconductor layer 120 forms the body diode 180.
Since SiC is a wide bandgap semiconductor, the body diode 180 has a relatively high turn-on voltage Vf of around 3 V (e.g., approximately 2.7 V) at room temperature and would cause a lot of loss. In this case, the turn-on voltage Vf is defined to be the electric potential Vsd at the first ohmic electrode 145 with respect to the one at the second ohmic electrode 170.
FIG. 10 shows the current-voltage characteristics and turn-on voltages of the body diode 180 at multiple different operating temperatures thereof. The turn-on voltage Vf of the body diode 180, which is obtained by making a tangential approximation on a curve representing its current-voltage characteristic, is as high as about 2.8 V at 25° C. A diode with such a high turn-on voltage is not practical. The higher the operating temperature, the smaller Vf. Also, as mentioned above, if the body diode 180 is used as a freewheeling diode, the degree of crystallinity of the semiconductor element 1100 will continue to decrease, the body diode 180 will have increased electrical resistance, and the loss will increase, too, which is a problem.
For that reason, it is difficult to replace the freewheeling diode element 1200 of the inverter circuit 1000 with the body diode 180.
The body diode 180 is a pn junction diode and is also an element that performs a bipolar operation. When the body diode 180 turns OFF, a reverse recovery current flows, thus causing some recovery loss. As a result, as there is a period in which the reverse recovery current flows, it becomes very difficult to switch the semiconductor element 1100 at high rates. In addition, since the switching loss increases, it becomes difficult to increase the switching frequency, too.
FIG. 11 is a circuit diagram illustrating a part of the configuration shown in FIG. 8 for illustration purposes. In FIG. 11, the DC power supply 2000 supplies power to an inductive load 2100 such as a motor. A high-side MISFET H and a low-side MISFET L are connected in series together. A controller 2200 that drives the high-side MISFET H and the low-side MISFET L outputs a gate drive voltage Vg1 to the high-side MISFET H and a gate drive voltage Vg2 to the low-side MISFET L, respectively.
The controller 2200 and the DC power supply 2000 together function as a “electric potential setting section” for setting the electric potentials of respective MOSFETs (i.e., semiconductor elements). And the semiconductor device shown in FIG. 11 is driven by that electric potential setting section.
Each of the currents I1 and I2 indicated by the arrows in FIG. 11 is supposed to have a positive value when flowing in the direction indicated by the arrow and a negative value when flowing in the opposite direction to the one indicated by the arrow, respectively.
Portions (a) through (e) of FIG. 12 show the operating waveforms of the circuit shown in FIG. 11 and illustrate a timing diagram showing voltages applied to, and currents flowing through, respective parts of the circuit when a current needs to be supplied to the inductive load 2100.
The respective gate drive voltages Vg1 and Vg2 for the high-side MISFET H and the low-side MISFET L are turned ON and OFF exclusively. In addition, a dead time Td1, Td2 is provided between Vg1 and Vg2 to prevent the high-side and low-side MISFETs H and L from turning ON simultaneously and causing a short-circuit breakdown.
In the initial state indicated by the timing diagram shown in FIG. 12, Vg2 is ON state to make a current flow in the path indicated by the arrow 96 shown in FIG. 11. Next, during the dead time Td1 after Vg2 has fallen to OFF state, current flows in the path indicated by the arrow 97 shown in FIG. 11. That is to say, the current flows through the freewheeling diode element that is connected anti-parallel to the low-side MISFET L. In that case, the current I1 has a negative value.
If the high-side MISFET H is turned ON while current is flowing through the freewheeling diode element that is connected anti-parallel to the low-side MISFET L, a voltage is applied to that freewheeling diode element that is connected anti-parallel to the low-side MISFET L. This voltage is a reverse voltage for the freewheeling diode element. That is why after a reverse recovery current has flowed through the freewheeling diode element that is connected anti-parallel to the low-side MISFET L along the path indicated by the arrow 95 shown in FIG. 11, that freewheeling diode that is connected anti-parallel to the low-side MISFET L turns OFF. More specifically, when the high-side MISFET H turns ON, the reverse recovery current flows from the high-side MISFET H through the freewheeling diode element that is connected anti-parallel to the low-side MISFET L as a sort of transient current, which is illustrated as a peak current 98. This reverse recovery current never flows through the inductive load 2100. However, as indicated by the arrow 95 in FIG. 11, the reverse recovery current is superposed on the current flowing through the high-side MISFET H, thus causing an increase in switching loss, a breakdown of the element due to overcurrent, and a lot of noise.
When the freewheeling diode element that is connected anti-parallel to the low-side MISFET L turns OFF, current flows along the path indicated by the arrow 94 in FIG. 11. Next, during the dead time Td2 after Vg1 has fallen to OFF state, current flows along the path indicated by the arrow 97 shown in FIG. 11, i.e., through the freewheeling diode element that is connected anti-parallel to the low-side MISFET L.
When the low-side MISFET L turns ON while current is flowing through the freewheeling diode element that is connected anti-parallel to the low-side MISFET L, a current flows along the path indicated by the arrow 96 shown in FIG. 11, i.e., through the channel of the low-side MISFET L. As a result, the initial state is recovered. It should be noted that although the high-side MISFET H and the low-side MISFET L turn ON and OFF at mutually different times, the reverse recovery current is also generated on the high side, and therefore, current does flow through the freewheeling diode element on the high side.
Next, the reverse recovery current of a pn junction diode will be described with reference to FIG. 13, in which curves (a) and (b) show variations in the amount of current flowing through a pn junction diode of Si (which is labeled as Si-PND). Specifically, the curve (a) shows the results obtained at 25° C. (Tj=25° C.) and the curve (b) shows the results obtained at 150° C. (Tj=150° C.).
As indicated by these curves (a) and (b), a pn junction diode has a period in which a reverse recovery current is generated, thus deteriorating the performance of the inverter circuit 1000 (e.g., interfering with the high-rate switching and increasing the switching loss). The magnitude of the reverse recovery current indicated by the 150° C. curve (b) is greater than what is indicated by the 25° C. curve (a). That is why the higher the temperature, the more seriously the characteristic of the Si-pn junction diode is affected.
On the other hand, the curve (c) shown in FIG. 13 shows a variation in the amount of current flowing through a Schottky barrier diode of SiC (which is labeled as SiC-SBD). As indicated by the curve (c), the magnitude of the reverse recovery current generated in that case is smaller than what is indicated by the curve (a) or (b). In addition, since the curve (c) shows both of the results that were obtained at 25° C. and 150° C., it can be seen that almost no reverse recovery current is generated in the SiC-SBD even at high temperatures. For that reason, a SiC-SBD, rather than Si-PND, had better be used as the freewheeling diode element 1200.